July 18, 2017
Construction work on the eastbound E6 motorway between Trondheim and Stjørdal in Norway is on track to meet the 2014 completion deadline. However, the project has had its challenges, not least in the construction of a 2,6 km road tunnel on the approach to central Trondheim. Geotechnical engineers and constructors were successfully overcoming the difficult ground conditions thanks to Atlas Copco's Elemex casing system.
To protect the city’s cultural heritage, the Trondheim authorities simply lifted up five endangered wooden building by a giant crane and carried away intact to a safe location. While this was under way, NCC, the project’s geotechnical engineers, concentrated on finding a way to stabilize the ground for the installation of retaining walls. With the site, consisting of layers of clay between 9 m and 23 m thick, as well as a groundwater level only about 1,5 m below the surface, all activities that could disturb the clay or lower the groundwater were strictly prohibited.
This was no ordinary routine job! Firstly, the planned tunnel path ran into 300 m of loose, watery clay, including 100 m of extremely sensitive “quick clay,” and secondly, the route was Close to a number of historical wooden buildings, perched on wooden piles. As a result, Hydraulic Impact Hammers could not be employed, as this would have created vibrations in the clay, possibly leading to disturbance and potential damage to the historical buildings.
The Norwegian roads administration, in cooperation with the contractors, decided to test a new method of using drilled casings equipped with an interlocking system along the length of the pipes. Extensive measuring was conducted including piezometer, total pressure and in clinometer tests. It soon became clear that Atlas Copco’s large diameter casing system Elemex was the only one capable of meeting all the requirements.
Consisting of a ring bit attached to a casing pipe and a pilot bit attached to an inner drill string, the Elemex bit drives a ring bit with gentle air flushing across the bit face. This enabled each pipe to be gradually “pulled” down through the clay until it reached bedrock. The pilot bit was then used to drill into the bedrock in the normal way. Once the pipe was securely installed in the bedrock, the inner drill string was removed leaving the pipe ready for cement injection.
“To meet the special demands on this job the Elemex system was a good choice. It was a continuous solution whereby we were able to get through the clay and into the bedrock in one operation. Once we got it right, it became routine.”
To drill the casings in place, two modified sheet-piling units were each equipped with Atlas Copco QL200 hammers. 610 mm casings and 674 mm oversized ring bits were used to lock the elements of each pipe, to easily slide into each other. The holes were drilled large enough for each new casing that followed. With this setup, a total of 349 interlocking pipes were installed to depths ranging from 11–33 m and 0,2–4,6 m in solid rock. The result was retaining walls that are both watertight and load bearing, horizontally as well as vertically.
Once up to speed, the Contractors was installing 14 casings per day at an average length of 20 meters and boring 1.5 to 4 meters in rock and, thanks to the vibro rotation method, resonance was completely eradicated. The cut-and-cover Trondheim tunnel was completed at the end of 2011 after which the five historical buildings that had previously been removed were replaced on their original sites. The tunnel was subsequently opened to traffic at the beginning of 2012 – right on schedule.
Kynningsrud Fundamentering and sub contractor Hallingdal Bergboring.
To efficiently drill casings through clay and bedrock while preserving the city’s cultural heritage
• Two modified sheet-piling units
• Two Atlas Copco QL200 DTH hammers
• 610 mm casings
• 674 oversized ring bits
• Atlas Copco Elemex system
• Vibro-Rotation method
• Easy to adapt and use Elemex with existing rigs eg. sheet-piling unit • Ideal for urban work m
• Made to drill through overburden and rock in a single operation
• Air-flow and flushings is diverted back through the pipe minimising damage to the hole and surroundings
• Straight and fast drilled holes
Elemex can drill faster, straighter and further while minimising any effect on the environment.